1. Field of the Invention
This invention relates to portable seismic signal source systems, and more particularly, to such systems of the vibrator or impulse type.
2. Description of the Prior Art
Transportable vibrator-type or impulse-type seismic signal sources are in wide use in seismic exploration. Examples of such devices are disclosed in U.S. Pat. Nos. 3,306,391; 3,550,719; 3,811,530; 3,905,446; and 3,295,630. In vibrator-type sources in particular, a vehicle such as a truck supports a large, heavy, substantially horizontal base plate that is connected by a vertical shaft to an inertia mass. A hydraulic linear actuator causes the base plate to move in reciprocating motion along a vertical axis against the reaction of the inertia mass. The base plate injects a swept-frequency vibratory signal train or "sweep" into the ground that lasts for several seconds (a "sweep cycle"). The frequency of the vibratory signal may be varied continuously throughout the sweep cycle, for example from 10 to 70 Hertz although frequencies from 5 to more than 80 Hertz may be employed. During the sweep cycle, a portion of the weight of the truck is applied to the base plate through isolating springs. The isolating springs are sufficiently compliant to permit the base plate, the vertical shaft connected thereto, the inertia mass and associated support structure to tilt a few degrees about the vertical axis with respect to the vehicle. A small amount of tilt with respect to the vehicle is necessary in order to allow the base plate to adjust itself to the local ground contour. So long as most of the area of the base plate remains firmly coupled to the ground the sweep signal will be reliably injected into the ground along a substantially vertical axis under the driving force of the linear actuator against the inertia
At certain frequencies around 14 Hz, the entire system becomes resonant. As is well known in the science of kinetics, in an undamped vibratory system, at frequencies in the region of the resonant frequency, a very small disturbing force will cause, theoretically at least, an infinite increase in the displacement of the oscillating mass. Hence, at resonance, particularly in rough terrain, the base plate and supporting structure violently dance or rock from side to side, fore and aft and in combinations thereof, about the vertical axis of the supporting structure. The violent rocking motion not only causes the base plate to become decoupled from the ground, thereby degrading the injected signal, but also causes severe structural damage to the entire vibrator assembly. There is a need therefore, for a restraining device to prevent the violent rocking motion of a vibrator apparatus.